Repolarization occurs after myocardial contraction. Electrocardiograms (ECGs) collected from electrodes placed on the surface of a patient's body can exhibit cardiac repolarization and, in particular, repolarization due to contraction of ventricular myocardium. Repolarization associated with normal ventricular contraction is typically referred to as a “T wave”. An ECG T wave is representative of a sum of repolarizations of many individual myocardial cells; hence, an ECG T wave exhibits little spatial information. However, where spatial repolarization heterogeneity exists among the many cells, an ECG T wave may exhibit some differences when compared to an ECG T wave from a more homogenous repolarization. In general, exhibited differences in ECG T wave morphology are referred to commonly as T wave alternans (i.e., alternating T wave morphology).
Various studies have linked the presence of T wave alternans to risk of ventricular arrhythmia, a major cause of sudden cardiac death. The link suggests that an increase in heterogeneity of ventricular myocardial repolarization indicates an increased risk of ventricular arrhythmia and possibly even an imminent onset of ventricular arrhythmia. Hence, information contained in an ECG T wave may possibly be used to prevent cardiac death.
Some studies have proposed implantable cardiac pacemakers that use T wave alternans detection. In general, such devices rely on intraelectrocardiograms (IEGMs) rather than surface ECGs to detect T wave alternans. Information exhibited by IEGMs differs from that of surface ECGs because in IEGMs, measurement electrodes are physically and/or electrically more intimately in contact with myocardial tissue. IEGMs typically include information same or similar to waveforms characteristic of cellular action potentials such as monophasic action potentials (MAPs), etc.
While the nature of information in ECGs and IEGMs differs, such devices typically seek to collect and analyze IEGM information to discern T wave alternans in a manner akin to that used for surface ECGs. For example, one study used a unipolar electrode configuration with an electrode in the right ventricular apex because repolarization alternans results from a spatially extended dispersion of ventricular repolarization and because the unipolar voltage vector covers a greater area of ventricular myocardium than standard bipolar recordings. Again, surface ECGs also measure a spatially extended dispersion of ventricular repolarization and have a voltage vector that covers a large area of the ventricular myocardium.
While such spatially extended measurements of ventricular repolarization may prove helpful in assessing risk or imminent onset of arrhythmia, a need exists for techniques that can measure and use local and/or spatial information. Various exemplary methods and/or devices described herein include use of local and/or spatial information.